1. Field of Invention
This invention relates in general to lasers, and in particular, to a device for initiating a laser beam using a turbojet engine plume or equivalent.
2. Description of Prior Art
Several types of lasers exist. In one type, a gas such as carbon dioxide, is maintained in a chamber at a very low pressure. By using light, heat, or electricity, molecules in the gas are excited. The atoms in the molecules have electrons which reach higher energy levels. When an electron of an atom drops to a lower energy level, the atom emits a photon. If an emitted photon strikes another atom when the atom is in an excited state, another photon will be emitted from the atom if the atom is of a type that emits photons of the same wavelength as the striking photon. Both photons then proceed in the same direction at the speed of light. If each of the two photons strikes an excited atom which emits photons of the same wavelength, then four photons will be travelling in the same direction.
In a laser, a fully reflective mirror is located on one side of the stimulated atoms and a partially reflective mirror is on the other side of a gas chamber. The photons in the gas chamber travel back and forth between the mirrors, striking excited atoms and gathering in number and thus energy exponentially with each reflection up to a maximum level. A small portion of the photon beam, typically about three percent, passes through the partially reflective mirror with each strike. The beam passes through the partially reflective mirror as light energy known as a laser.
A problem in achieving higher energy laser beams is in stimulating and maintaining the molecules in a stimulated condition. If a photon strikes an atom that is not in an excited energy condition, the atom will absorb the photon and become stimulated to a higher energy level. However, a photon will not be emitted. The photon beam bouncing between the mirrors loses energy for each photon captured. A dense gas potentially can be used to create a higher energy laser beam because it has more molecules than a less dense gas, and thus potentially could produce a larger number of photons travelling between the mirrors. However, with a dense gas it is difficult to maintain stimulation of molecules. Too many molecules will not be at an exited state, thus capturing too many photons. Generally, lasers use only a gas with only one type of molecule and at a very low pressure, much less than atmospheric. In this manner all of the emitted photons will be at the same wavelength.
In a gas dynamic laser, the pressure is somewhat higher, but typically still less than one PSI. The gas dynamic laser utilizes a fuel, a burning chamber, and igniter to ignite the fuel. The burning creates a hot gaseous stream, which contains a large number of excited molecules. The hot gaseous stream is drawn and exhausted by a positive exhaust system into the atmosphere. The mirrors are placed across the flowing gaseous stream, creating a reflection path for photons emitted from excited atoms struck by other photons. The photons in this beam are filtered spectrally so as to filter all but a single wavelength. A small portion of the beam passes through the partially reflective mirror.
In a dynamic gas laser, a higher energy laser can potentially be achieved than with a static gas type. However, it is an expensive, large and complex system. Also, energy levels achieved to date have not been sufficient so as to be able to burn metal, unless the beam dwells on the metal for a considerable time, such as one second. Gas dynamic lasers are not yet feasible for weapons on aircraft because of the large size and because of the inability to burn metal almost instantly.